What is A4 paper?

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A4 paper is a format of paper that follows the ISO international standard that is used in a large part of the world. Its dimensions are 210 by 297 millimeters or, equivalently, 8.27 by 11.69 inches, making it somewhat longer and narrower than the North American 8.5-by-11-inch letter-sized format.

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A4 is the standard office paper in many countries. The International Organization for Standardization standard defines a whole range of paper formats. The A series contains formats A10, or small, up to A0, or large, plus two extra-large formats: 2A0 and 4A0. The advantage of the standard is that the length-to-width ratio is always constant and the dimensions of two sheets placed side-by-side correspond to the next size in the series. For example, an A3 measures 297 x 420 millimeters. This facilitates rescaling and fitting multiple pages per sheet. Two other series, B and C, are also defined and used when making books and envelopes.